The invention relates to a method for calendering a paper or paperboard web and to a device for implementing the aforementioned method.
In connection with papermaking, after the drying of the paper web, the web is subjected to calendering as a normal finishing step. There are many calendering methods, but it is common to all of them that the web is led through one or more nips formed between two surfaces, typically between rotating roll surfaces. The purpose of the calendering is to improve the quality of the paper by pressing it to a particular standard final thickness by affecting the density of the paper and by smoothening its surface, to achieve a desired paper gloss and/or smoothness. In other words, the calendering affects both the visual and the structural properties of paper.
The calendering roll can be a hard-faced heated thermo roll, a soft-faced variable-crown roll, a soft polymer roll, or a soft long-nip roll. Typically, one roll in a calendering nip is a hard-faced thermo roll and the other roll is one of the above-mentioned soft rolls. In multinip calenders, the rolls forming a nip may also be two soft-faced rolls.
Variable-crown rolls and polymer rolls are coated with a soft polymer coating which is normally made of an epoxy with a poor thermal stability. Consequently, in a nip in which one roll is a heated thermo roll and the other one is a roll coated with a soft coating, one must take care that the coating of the coated roll does not touch the thermo roll, which could result in a damage of the coating. A damaged roll must be replaced with a new one, which causes a break in the operation of the calender and increases the maintenance costs of the device.
At present, the coating of coated rolls is protected from the contact with the thermo roll e.g. by setting the width of the paper web precisely according to the width of the coated roll as well as by bevelling the edge areas of soft coatings to prevent the coating from touching the thermo roll outside the paper web. It is also known to calender the web in excess width, wherein the width of the web exceeds the length of the coated roll in the axial direction, wherein the outer edges of the web remain outside the nip and are thus not calendered at all. These edge areas can either be cut off or they can be calendered separately in an edge calendering step. The cutting of the edges requires that space-consuming edge cutters are placed on both sides of the web width in the calender. Furthermore, the strips cut off from both edges of the web, which are conveyed to a pulper, increase the quantity of waste from the paper machine. The cutting of the edge areas of the web after the calendering is disclosed in GB 2218434.
Edge calendering is performed either before or after the actual calendering. In U.S. Pat. No. 6,189,442, before the calendering of the rest of the web width, the edge areas of the paper web are calendered in a nip with a separate counter roll extending in its length across the whole width of the web, and considerably shorter edge calendering rolls corresponding to the width of the edges of the web extending beyond the length of the actual soft-faced calendering roll, at each edge of the web. One problem in this arrangement is the fact that the roll arrangements required by the edge calendering consume space in the calendering device.